Editor—We have the following concerns with Antonioli and Reveley's study of animal facilitated depression with dolphins in the treatment of depression.1

The study was small (30 patients), only 13 people from the animal care programme and 12 from the outdoor nature programme completing the study. The authors used a conservative measure to estimate the number of patients required for this study, and even then fell short by 17%.

It is difficult to fathom how clinical raters can be blinded to the treatment hypothesis.

The authors did not mention the ethnic group of the participants.

Only people who could go to Honduras were able to take part in this study. Most people with mild depression will not be able to take a three week holiday.

The findings are not generalisable as most people would not be able to afford to go to such a location and swimming and snorkeling are not necessarily favourite pastimes.

There may be a vacation bias. If the study group sponsored the three week trip to Honduras we suspect that a free three week trip to a seaside location in Latin America would in itself be a powerful antidepressant.

There may be a disappointment bias. If the control group were aware of the potential to be in a group where they could interact with dolphins for two weeks before they reached Honduras, they could be disappointed in missing out. People with depression perceive disappointment more intensely.

The study could have been done in a more natural and plausible setting with common pets such as dogs or cats for a fraction of the cost.